Thursday, November 5, 2009
R1a1a7: a signal of Slavic expansions from Poland
Nature has just published a paper on the discovery of a European-specific subclade of R1a, defined by the M458 mutation and classified as R1a1a7. It's a long-awaited and ground-breaking result, because structure within R1a has been poorly understood to date, despite the fact that it's one of the most dominant Y-chromosome haplogroups in Europe and Asia. The data included in the study point to what is now Poland as the most likely place of origin for R1a1a7. Here's a nice map...
However, as per the figure above, the authors claim that R1a1a7 originated about 10.7K years ago. This, they say, makes it a likely signal of population movements carrying agriculture from Central Europe to what is now Ukraine and European Russia during the Neolithic. Unfortunately, that doesn't make any sense, because R1a1a7 is very rare in Scandinavia, which was largely populated from Central Europe after the Ice Age. Indeed, recent work on the population movements around the Baltic suggests that both R1a and I1a moved up from Germany and Poland into Sweden.
So something's not quite right there, and I think what happened was that the authors grossly overestimated the age of R1a1a7. They did this by using the so called evolutionary effective mutation rate in their Y-STR calculations. This methodology is commonly used by scientists, but it's generally frowned upon by hobby genetic genealogists, who prefer the so called germline mutation rate. The germline rate produces estimates which are about a third of those obtained with the evolutionary effective rate, but they always seem to make more sense. That's been my impression from the results I've seen over the years anyway.
Let's assume then, that R1a1a7 has only been around for 4,000 years or less. If so, that would make it a perfect candidate for a paternal marker of the proto-Slavs, who probably originated in what is now Southern Poland and began expanding during the late Iron Age. Their descendants eventually settled across much of Central, Eastern and Southeastern Europe, but rarely in Scandinavia, which correlates well with the current spread of R1a1a7.
The paper also claims that it's unlikely there were any major post-Neolithic population movements from Eastern Europe to Asia, because R1a1a7 is today basically missing east of the Urals. But what if a different, as yet undiscovered subclade of R1a migrated from Europe to Asia before R1a1a7 had a chance to expand? Ancient DNA results do suggest that Europeans carrying R1a migrated east via the Russo-Kazakh steppe as far as South Siberia during the Bronze Age (see here), but this information was ignored by the authors.
So well done on finding the new R1a subclade, but there's obviously something off about its age/expansion estimates. When will that change I wonder?
Peter A Underhill et al., Separating the post-Glacial coancestry of European and Asian Y chromosomes within haplogroup R1a, European Journal of Human Genetics advance online publication 4 November 2009; doi: 10.1038/ejhg.2009.194
T. Lappalainen et al., Migration Waves to the Baltic Sea Region, Annals of Human Genetics, Volume 72 Issue 3, Pages 337 - 348, doi: 10.1111/j.1469-1809.2007.00429.x
Update: R1a1a7 is now known as R1a1a1b1a1. See here.